This research is a case study of two teachers with whom I worked to explore a constructivist philosophy of learning in mathematics and the impact of this intervention on their thinking and actions in the teaching of mathematics. The two teachers who participated in this study were from a private school in Karachi where they teach Maths in Grades vii & viii. Because my intention was to compare the teachers' initial perceptions and actions to their perceptions and actions at the end of my intervention, two sets of observations and interviews were carried out. Data from these and other sources were triangulated to get valid information about the changes that occurred in the teachers' perceptions and actions as a result of the intervention. The findings of the study indicated that the intervention had a positive impact on both the teachers as well as the students in whose classes the research was carried out. Many problems and constraints associated with the implementation of teaching based on a constructivist philosophy of learning were also identified during the research.